1. Field of the Invention
The invention relates to control of a vehicle on which an automatic transmission is installed, and in particular to control of the vehicle for eliminating a state in which air is present in a hydraulic fluid supplied into the automatic transmission when the hydraulic fluid has a low temperature.
2. Description of the Related Art
In an automatic transmission, a hydraulic fluid stored in an oil pan is sucked up by an oil pump through a strainer, and is supplied to each constituent component of the automatic transmission. In the automatic transmission of this type, if the suction amount of the hydraulic fluid through the strainer increases because the viscosity of the hydraulic fluid is high, or the engine speed is high, for example, the amount of the hydraulic fluid in the oil pan is reduced, and the strainer may be exposed to above the oil level, thus causing a problem that air is likely to be mixed into the hydraulic fluid when it is sucked through the strainer. Similarly, when the oil level of the hydraulic fluid in the oil pan changes due to the inertial force applied to the hydraulic fluid under certain running conditions of the vehicle, the strainer may be exposed to above the oil level, thus causing a problem that air is likely to be mixed into the hydraulic fluid when it is sucked through the strainer. If the hydraulic fluid that contains air is supplied to a friction device that is engaged so as to establish a gear position of the automatic transmission, a sufficiently high hydraulic pressure needed to engage the friction device cannot be supplied to the friction device, and slipping may occur in the friction device.
As one example of hydraulic control system for solving the problem as described above, a hydraulic control system for an automatic transmission as described in Japanese Patent Application Publication No. 7-174216 (JP-A-7-173216), for example, is arranged to prevent the engine from stalling due to uneven distribution of the oil in the oil pan upon abrupt braking. The hydraulic control system is employed in the automatic transmission having a torque converter equipped with a lock-up device capable of directly coupling an input element to an output element thereof, and includes a line pressure actuator for regulating a hydraulic oil pressure delivered from an oil pump of the automatic transmission so as to develop a line pressure, and a line pressure controller for controlling the operation of the line pressure actuator so as to form a line pressure corresponding to vehicle running conditions, based on input from a running condition detector. The hydraulic control system is characterized in that an exposed-state detector for detecting a state in which an inlet of the oil strainer provided on the inlet side of the oil pump is exposed to above the oil level, and that the line pressure controller executes control for increasing the line pressure while the state in which the inlet of the oil strainer is exposed to above the oil level is detected by the exposed-state detector.
With the hydraulic control system for the automatic transmission constructed as described above, even when the oil in the oil pan collects on one side during braking, and the inlet of the oil strainer is exposed to above the oil level, resulting in a reduction in the pressure delivered by the oil pump, the line pressure controller controls the operation of the line pressure actuator so as to increase the line pressure, so that the line pressure is not reduced as much as the delivery pressure of the oil pump. Accordingly, the release pressure supplied to the lock-up device so as to release the lock-up clutch of the torque converter in response to the braking action is kept at a higher level than the apply pressure, and the engine is prevented from stalling due to failed release of the lock-up clutch which would be otherwise caused by reduction of the release pressure.
In the hydraulic control system for the automatic transmission as described in the above-identified publication, however, the exposed-state detector needs to be provided for detecting the state in which the inlet of the strainer is exposed to above the oil level, resulting in an increase in the number of constituent components of the automatic transmission and increased cost.